1. Technical Field
This invention relates to microwave technology and, more specifically, to microwave susceptors; i.e. materials capable of generating thermal energy from microwave energy. Although solid materials have long dominated susceptor applications, liquid susceptors are gaining recognition as useful alternatives.
This invention relates to the usage of various liquid substances which possess low volatility and high stability as permanent and reusable microwave susceptors. It employs such materials to convert microwave energy to thermal energy in the form of stored heat suitable for a variety of applications in and out of the microwave. 2. Description of the Prior Art
It is well known among practitioners of microwave cooking that, speed of preparation notwithstanding, microwave ovens produce results which are quite different from those obtained in conventional ovens. Microwaves heat food essentially throughout by acting upon microwave susceptible components such as water, salts, sugars and the like. Food components which are less microwave interactive do not absorb microwave energy as readily, but heat up by their close proximity to and admixture with receptive components in a constant process of thermal equilibration. In contrast, conventional ovens heat foods by conduction, radiation and convection from the outside in. This method of heating produces surface effects such as browning and crisping which are often desirable but not attainable in an all-microwave oven.
A few microwave ovens now offer added radiant or convective heat in attempts to simulate conventional ovens. Manufacturers of microwave cookware are also trying to address this need by specialized cooking utensils, described as browning grills or skillet browners, which feature microwave susceptible surfaces made of ferritic materials and the like, as described in U.S. Pat. Nos. 4,496,815 and 4,542,271. Similar considerations are being given to the development of cook-in packaging for foods by providing extra heat from microwave susceptors to promote crispness. The latter is exemplified by a variety of crisping boards, as cited in U.S. Pat. Nos. 4,267,420 and 4,258,086, which carry microwave susceptors based on vacuum-metallized or metal-sputtered polyester film, for such foods as french fries, fish sticks, pizza and the like.
It is clear that much attention has been given thus far to solid microwave susceptor materials. Solid materials, however, be they natural or man-made, cannot address all needs. Among microwave susceptors which are liquid, water is best known as the major microwave susceptible component of foods. Water has also been used as a supplemental source of microwave heat in various cooking or heating applications, as described in U.S. Pat. Nos. 4,501,946, 4,439,656, 4,316,070 and 4,283,427. However, water and other volatile liquids cannot practically serve as reusable sources of thermal energy for a number of reasons. First of all, they boil at temperatures which are of limited interest. Secondly, long before their boiling point is reached, they begin to evaporate, therefore being in need of frequent replacement. Thirdly, when confined, they are apt to build up pressures which cannot be safely controlled. Thus, the useful temperature range of water and aqueous liquids is essentially defined by the volatility and boiling point of water.
U.S. Pat. No. 4,795,649 by Kearns et al addresses the problem by the use of non-aqueous, organic liquids as sources of heat in a double-boiler cooking method. The liquids are microwave susceptible and relatively non-volatile capable of reaching temperatures not attainable commonly in microwave cooking. The liquids cited in the patent are essentially aimed at one specific application. However, microwave susceptible fluids are available in such a wide variety of dielectric and other physical properties that their full potential is yet to be realized not only for cooking but, even more so, for non-food applications.
In the field of health-care, for instance, there are many hot and cold compresses to choose from. Several portable compresses contain fluids or gels which must be precooled or preheated before use, as described in U.S. Pat. Nos. 3,885,403 and 3,780,537. Others derive their thermal effects from physical phenomena such as solution or crystallization as cited in U.S. Pat. Nos. 3,874,504 and 4,462,224. With the advent of microwave technology, some of the newer compresses are claimed to be preheatable in the microwave. They apparently contain gelled or relatively volatile aqueous compositions which can be damaged or rendered less effective by repeated use or excessive preheating. Since they require elaborate precautions and occasional reconditioning or regeneration, there is clear need for microwave reheatable devices which are safe, more forgiving and simpler to use.
In the area of leisure and recreation, reusable cold cubes have been available for many years, comprising fluids permanently encased in freezer-safe and puncture-resistant containers. There are no counterpart devices in the market for heating items or maintaining them hot, which use encapsulated liquids as sources of heat; that is, reusable "hot cubes". With the advent of microwave technology and the availability of liquid susceptors such items are now closer to reality.
Accordingly, the object of this invention is to provide greater opportunity for employing liquid susceptors as permanent and reusable sources of microwave generated thermal energy for food and non-food applications. A further object of the invention is to provide the microwave generated thermal energy in the form of stored heat available for application in the oven, during the microwaving, or outside the oven, after the microwaving.